Here you can see a reference chart showing the inner diameter of each bangle size. If you don't already have a bangle to take a measurement from, this is how you can measure your hand to find your ideal fit: Tuck your thumb into your palm and wrap a measuring tape around the widest part of your hand, i.e. from your pinky base knuckle around to your thumb base knuckle. The fit should be snug but not too tight. You will now have your inner circumference measurement, which can be divided by Pi (3.14) to give your ideal inner diameter measurement.